1. Field of the Invention
The present invention relates to a rubbing apparatus for orientating a film formed on a transparent electrode of a liquid crystal display device, and a method for manufacturing liquid crystal display devices with using the rubbing apparatus.
2. Description of the Prior Art
A manufacturing method of a liquid crystal display device in the prior art will be first depicted hereinbelow.
A conventional rubbing apparatus is constituted of a fixing stage for fixing a transparent electrode substrate having an orientation film (alignment polymer) formed on the surface thereof, and a movable part which moves and rotates a drum with a cloth of rayon, nylon, cotton or the like wound therearound along the fixing stage. When an orientation film is to be rubbed by the rubbing apparatus, after such conditions as the rotating frequency cf the rubbing roller (rotary drum), moving speed of the substrate, pressing amount to the orientation film (pressing force), etc. are preliminarily set, the orientation film on the surface of a transparent electrode substrate is brought in touch with the rubbing cloth of the rotating rubbing roller, thereby to rub the orientation film (Japanese Patent Laid-Open Publication Tokkaisho No. 55-143525 (143525/1980)).
In the conventional rubbing method as above, it is quite difficult to set the rubbing conditions and it is hard to uniformly rub the orientation film because the distance between the surface of the orientation film and the rubbing roller with the rubbing cloth (that is, pressing amount of the rubbing roller against the orientation film), the revolution number of the roller, the moving speed of the substrate of the diameter of the roller may be varied even after these rubbing conditions are set. The surface of the orientation film would be flawed (rubbing flaw) in the worst case.
If a liquid crystal display device is constituted by using the flawed transparent electrode substrate, since the substrate at the part of the flaw acts in a different manner to the electric field, an abnormal area of a threshold value is generated along the flaw. Even if the orientation film is rubbed on condition that the rotating frequency of the roller, moving speed of the substrate, pressing amount to the film, and the diameter of the roller are constant, namely, when the rubbing conditions are kept constant, it is not sure that the orientation film is rubbed with the constant rubbing strength at all times, due to the change of the thickness of the glass substrate, the difference per lot of the rubbing cloth, the change with time of the mechanical accuracy, etc. Although it is not completely made clear how the liquid crystal molecules are orientated by rubbing, generally, it is so considered that the polymer chains in the vicinity of the surface of the orientation film are orientated when the shearing force is applied to the surface of the orientation film through rubbing, and the liquid- crystal molecules are aligned in the direction of the orientation of the polymer chains. Therefore, the orientation state of the surface of the orientation film works important influences upon the orientation of the liquid crystal molecules. Although a double refraction method (J.M. Geary, J.W. Goodby, A.R. Kmetz and J.S. Petel, J. Appln. Phys., Vol. 62, No. 10 (1987)) is known as a method to quantitatively measure the orientation state of the rubbed orientation film, there has been no proposal yet to measure a large area at many points with high speeds. Moreover, the suitable orientation state has not been determined for the rubbed orientation film. In the absence of a sufficient evaluating method of the orientating stage before assembling of a liquid crystal display panel, it is impossible to detect whether or not the rubbing is performed well before the panel is actually assembled, thereby increasing the number of defective products. In other words, when the rubbing state of the orientation film is not uniform within the substrate surface, the pre-tilt angle of the liquid crystal molecules is changed, and consequently the threshold value becomes irregular, that is, the display irregularity (color irregularity) is caused in the panel, resulting in the deterioration of the display quality.